HAL USART DMA buffer reset method?

https://community.st.com/s/question/0D50X0000BFxPVhSQN/read-and-write-bluetooth-communication-byte-format

So more of a Synchronization issue.

The code in your other thread receives data into the buffer twice, and blocks (I assume)

Doing one byte at a time with DMA seems overkill.

Blocking doing 5 bytes and ping-pong between send/receive looks prone to lose data. You need to use DMA/IT versions that can function concurrently, and manages how much data comes in and goes out.

Fixed length receive really requires a continuous stream of data.

I have found this to be

void initUart1RxDMABuffer(void) {
 if (HAL_UART_Receive_DMA(&huart1, (uint8_t *)Usart1RxDMABuffer, U1RxBufSize) != HAL_OK)
 {
 sprintf(string, "initUart1RxDMABuffer Failed\n");
 puts1(string); 
 }
 else
 {
 sprintf(string, "initUart1RxDMABuffer OK!\n");
 puts1(string);
 }
}
char peekRxU1(int offset) {
 // offset of zero returns the next byte to be read
 int relativePtr = (U1RxBufferPtrOUT + offset) & (U1RxBufSize - 1); 
 char PeekRx = Usart1RxDMABuffer[relativePtr]; // just looking Don't increment pointer
return PeekRx;
}
char readU1(void) {
 char readByte = Usart1RxDMABuffer[U1RxBufferPtrOUT++];
 if (U1RxBufferPtrOUT >= U1RxBufSize) U1RxBufferPtrOUT = 0;
 return readByte;
}
char readableU1(void) {
 U1RxBufferPtrIN = U1RxBufSize - huart1.hdmarx->Instance->CNDTR;
 return U1RxBufferPtrIN - U1RxBufferPtrOUT;
} 
void clearRxBuffer(void) {
 U1RxBufferPtrIN = U1RxBufSize - huart1.hdmarx->Instance->CNDTR;
 U1RxBufferPtrOUT = U1RxBufferPtrIN;
}

the best way:

I wrote it

I use this on the F7, just looking over the code I sent you, in Peek(), there is a requirement that the Rx buffer size is 2^N

char readableU1(void) {
 U1RxBufferPtrIN = U1RxBufSize - huart1.hdmarx->Instance->NDTR;
 return U1RxBufferPtrIN - U1RxBufferPtrOUT;
} 

uint32_t U1RxBufferPtrIN ;

uint32_t U1RxBufferPtrOUT=0;

make your buffer size 2^N like 256 or 512 or 1024 or 2048 or 4096 bytes

Yes, use clear clearRxBuffer() to make the U1RxBufferPtrOUT equal to the DMA pointerIN , U1RxBufferPtrIN

then when you do readableU1() there return value will be Zero...